WLANs utilize RF signals or light signals to connect mobile devices to each other or to a centralized gateway and transmit data between the physical devices. In 1997 the IEEE published standards for WLANs under the title of 802.11. The 802.11 standards contain different protocols that may use unlicenced 2.4 GHz and 5 GHz radio bands to transmit packet data. IEEE 802.11 enables mobile stations (e.g., endpoints) to communicate through a wireless network interface card directly with each other or with other stations through an access point. An access point (AP) is a centralized gateway providing message and power management and access to an external LAN and/or the Internet. IEEE 802.11 access products are sold with personal computers (e.g., wireless NICs), computer peripherals, print servers, and mobile devices such as laptops and personal digital assistants (PDAs).
There are a plurality of 802.11 standards that each use different frequency bands and have varying data transmission speeds. The original IEEE 802.11 standard supported wireless interfaces operating at speeds of up to 2 megabyte per second (Mbps) in the 2.4 GHz radio band. By using different modulation techniques, IEEE 802.11b raised the data transmission rates to 11 Mbps, while 802.11a supports up to 54 Mbps transmission rates at a 5 GHz frequency. The IEEE 802.11g is developing standards for data transmission rates of 54 Mbps at the 2.4 GHz frequency.
WLANs under 802.11 use media access control (MAC) protocols to transmit between wired and wireless devices. Each wireless network card is assigned a MAC address used to identify the station. The basic protocol of an IEEE 802.11 network is the Basic Service Set (BSS), which is merely a number of endpoint stations that communicate with one another. The access to wireless networks is controlled by coordination functions. The distributed coordination function (DCF) provides access similar to Ethernet CSMA/CA access. The DCF determines if the RF link between devices is clear prior to transmitting. Stations use a random backoff after every frame to avoid collisions. Endpoint stations provide MAC Service Data Units (MSDUs) after detecting no current transmissions. The MSDUs functions to transmit data frames to the proper endpoint station.
Under the DCF access method of 802.11, each MSDU transmission incurs an overhead that includes a distributed interface space (DIFS) duration, a backoff interval, a Physical Layer Convergence Procedure (PLCP) preamble, a PLCP header, a MAC header, a SIFS duration,a dn an acknowledgment (ACK) time (which comprises a PLCP preamble, a PLCP header, and the ACK MPDU). Such an overhead amounts up to 764.2 μs on an 802.11b PHY with long PLCP preamble an 11 Mbps data rate.